This invention relates to arrangements for constructing semi-membrane tanks for liquefied natural gas (LNG) and the like.
In many conventional manufacturing methods for membrane-type LNG tanks installed in ships or other permanent support structures, partial tank sections are separately manufactured at an off-site location and are transported together with fixtures or assembling devices for separate installation in a ship""s cargo hold or other permanent support structure where they are welded to other partial tank sections in sequence until the entire tank has been assembled in the permanent support structure. During installation temporary staging and support bracing is required and access to the space between the outside of the tank and the ship""s hull must be provided while the sections are being welded together and tank insulation applied. In addition, internal pumps, piping and tank monitoring systems must be installed before final tank closure and testing. Furthermore, allowance must be made for thermal contraction and expansion of the tank with respect to the ship""s hull or permanent support structure.
Such erection and installation of a membrane-type LNG tank structure piece by piece within a ship""s hull results in a complicated ship design and an extended shipbuilding schedule. Moreover, when there is a close fit between the LNG tank and the ship""s inner hull, access to certain parts of the tank is restricted, and the number of personnel who can be given access to complete the tank erection process is limited. Furthermore, an attempt to build the ship and construct the LNG cargo tank within the ship simultaneously complicates both the ship and tank construction and restricts access to the necessary building resources such as cranes, welding, ventilation and the like for one or the other activity.
Proposals have been made heretofore to construct an entire membrane-type LNG tank outside a vessel or support structure in which it is to be installed and then transfer the completed tank to the vessel but that procedure gives rise to many problems which have not been solved satisfactorily. For example, the prior art does not satisfy the need for complete support of the tank structure while it is being assembled outside the vessel and while it is being transported to and installed in the vessel without requiring removal of temporary support components from the interior of the tank after installation.
The Yamamoto U.S. Pat. No. 3,861,021 discloses a method for constructing a double-membrane type LNG tank in which the entire tank is supported from a platform suspended from a crane during construction so that the tank is not subjected to gravitational effects as a result of its own weight. A temporary internal supporting structure is provided within the tank to support the top and bottom walls of the tank during construction and, as the tank is being installed in a vessel, a vacuum is applied to the space between outer and inner membrane walls of the tank to rigidify them.
The Cuneo et al. U.S. Pat. No. 5,727,492 discloses a membrane-type LNG tank and containment system for installation in an LNG cargo ship which can be constructed either within a ship or other final support structure or outside the ship or final support structure and can thereafter be lowered into place using internal supports to minimize the complexity of external rigging frames.
The Secord et al. U.S. Pat. No. 4,173,936 discloses an arrangement for supporting a membrane-type tank from the hull of a ship using load-bearing insulating blocks.
Accordingly, it is an object of the present invention to provide an integrated tank erection and support carriage arrangement for semi-membrane prismatic LNG tanks which overcomes disadvantages of the prior art.
Another object of the invention is to provide an integrated tank erection and support carriage arrangement which facilitates manufacture, assembly and installation of semi-membrane prismatic LNG tanks in ships or other permanent support structures.
These and other objects of the invention are attained by providing a carriage arrangement having top and side walls for supporting the top and side walls of a semi-membrane prismatic tank, along with a pipe tower extending into the tank from the support carriage and arranged to support the bottom wall of the tank during construction and installation while allowing for thermal expansion and contraction of the tank in use.